Cheng Fluid Systems, Inc.
United States
ph: +1(650) 559-9292
sales
Check valves, by the very nature of their design, respond to flow and pressure disturbances such as turbulence in the upstream piping system. This can result in the disc oscillating back and forth on the pin support. When a check valve is close-coupled to an upstream elbow, the turbulence becomes severe and the oscillations are of a large enough amplitude that the disc continually moves, may bang against the stop, and the pin eventually fails.
In check valve systems with upstream close-coupled elbows, one can eliminate harmful high amplitude pressure bursts (a) by installing a CRV® (b) upstream of the elbow.
Experimental results of check valve dynamics with and without a CRV® are shown above. (a) shows pressure measurements as a function of time with the elbow-check valve combination only (Without the CRV®). The high amplitude pressure bursts are damaging to the check valve and a result of the oscillating check valve disc and the turbulence generated by the elbow.
With a CRV® mounted in front of the elbow-check valve combination, turbulence generated by the elbow is eliminated. (b) shows an absence of high amplitude pressure bursts. This results in minimized wear on check valve pins, extended life, reduced vibration and noise, and better sealing because or pin integrity.
Flow disturbances emanating from the upstream elbows cause problems in the check valve. Most manufacturers of swing check valves recommend up to 10 diameters of straight pipe before the inlet of their valves. They also point out that swirl due to out-of-plane elbows will have a detrimental effect on flow and maintenance. Flow separation with shed vortices from the elbow and trailing edge of the valve disk, as it falls back into the stream due to pressure fluctuations, creating noise. The valve disk oscillating back and forth in the flow also sets up a stereo speaker-like pressure fluctuation. Reduced average flow through the valve will result from these elbow flow distortions. Another effect are pressure spikes that occur in the flow to the check valve due to localized matching of the separated flow signatures from the elbow and valve disk which will cause increased valve wear and pipe vibration amplitudes.
Check Valves Problem: | CRV® Benefits: |
Check valve chatter with close coupled upstream elbow | Eliminate elbow induced flow turbulence in check valves |
Disc pin wear and breakage | Extend pin life |
Poor sealing due to pin wear | Improve sealing life |
Solution: The Cheng Rotation Vane (CRV®) provides a uniform velocity profile at the elbow outlet and eliminates the turbulence and shed vortices which produce the vibration and loss of performance. A CRV® before the elbow preceding the check valve, will provide passive damping, due to reverse flow separating off of the trailing edge, to the valve closing pressure wave thus reducing water hammer damage to the pump and piping.
CRV® Potential Benefits:
Increase flow through the check valve due to an average wide open position.
Reduce check valve wear and replacement.
Reduce pump, piping and valve vibration.
Check Valves Downstream of Elbows
Because of the separated flow and large scale turbulence downstream of a plain elbow, it was anticipated that a downstream check valve would be excited to an oscillatory response, including banging against the stops.
An investigation showed that in addition to the response of the check valve to the elbow-generated turbulence, the flow around the oscillating check valve disc sets-up downstream turbulence having its own signature. The subsequent mixing of these two regions of flow turbulence was found to develop temporal resonances creating discrete strong pressure disturbances that propagate upstream past the check valve and further influence its motion. Insertion of a CRV® in front of the elbow eliminated the check valve disc oscillation and these traveling waves.
Our flow conditioners are custom made to specifically address each piping issue offering the greatest achievable flow correction.
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Cheng Fluid Systems, Inc.
United States
ph: +1(650) 559-9292
sales